[llvm] [VPlan] Don't cost FOR splice if unused in legacy cost model (PR #131486)

[Luke Lau via llvm-commits]

================
@@ -6541,6 +6541,11 @@ LoopVectorizationCostModel::getInstructionCost(Instruction *I,
       // TODO: Consider vscale_range info.
       if (VF.isScalable() && VF.getKnownMinValue() == 1)
         return InstructionCost::getInvalid();
+      // If a FOR has no users inside the loop we won't generate a splice.
+      if (none_of(Phi->users(), [this](User *U) {
----------------
lukel97 wrote:

Oh good point, adding a dead binary op does indeed cause another assertion failure, I've added a test for it.

It looks like a dead use of the FOR isn't enough to trigger planContainsAdditionalSimplifications on its own because the legacy cost model will detect it as dead and return true for skipCostComputation.

So I've moved the check from the legacy cost model to planContainsAdditionalSimplifications to check for VPFirstOrderRecurrencePHIs without any VPInstruction::FirstOrderRecurrenceSplice uses.

https://github.com/llvm/llvm-project/pull/131486